Curd and sour substances should not be kept in brass and copper vessels because these metals can react with acidic foods, leading to potential health hazards. The main concern is the leaching of toxic metals into the food or liquid being stored, which can contaminate the food and make it unsafe for consumption. Here are the reasons why:

1. Acidic Reactions: Curd and sour substances contain acids (such as lactic acid in curd) that can react with the copper and zinc present in brass or copper vessels. This reaction can cause the release of these metals into the food, leading to metal poisoning.
2. Copper Toxicity: Copper is an essential mineral needed by the body in small amounts, but excessive intake of copper can be toxic. Consuming food that has been in contact with copper vessels can result in copper toxicity symptoms such as nausea, vomiting, stomach cramps, and diarrhoea.
3. Brass Composition: Brass is an alloy made primarily of copper and zinc. The zinc content in brass can also react with acidic foods, causing zinc to leach into the food. High levels of zinc can cause health problems, including gastrointestinal issues.
4. Corrosion and Contamination: Over time, the acidic nature of curd and sour substances can corrode the surface of brass and copper vessels, increasing the likelihood of metal leaching.

To avoid these potential health risks, it’s best to use stainless steel, glass, or food-grade plastic containers for storing curd and other acidic foods. These materials are inert and do not react with the food, ensuring that there is no contamination and the food remains safe to consume.